Color method for detecting cracks in metal bodies

ABSTRACT

A nondestructive color method for detecting cracks in a metal body by selective corrosion of the surface portion within the cracks. Specifically, a color-forming aqueous acid indicating solution containing halide ions and a color-forming indicator is applied to the surface of the body to be tested. The solution is formulated so that it does not corrode to any significant extent the open surface portion of the body but is sufficiently reactive to corrode the surface portion within cracks resulting in the formation of metallic ions. The color-forming indicator is of the type which reacts with the thus formed metal ions to form a distinctive colored compound at the crack locations.

United States Patent Coffin, Jr. et al.

[ 51 Mar. 28, 1972 [54] COLOR METHOD FOR DETECTING CRACKS IN METAL BODIES [72] Inventors: Louis F. Coffin, Jr.; Lyman A. Johnson,

both of Schenectady, NY.

[2]] Appl. No.: 889,696

3,490,873 1/1970 Cor] ..23/230 2,007,285 7/1935 Schanffele ..23/230 3,425,950 2/1969 Derbyshire, Jr.

Primary ExaminerMorris O. Wolk Assistant Examiner--Elliott A. Katz Attorney-Charles T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7] ABSTRACT A nondestructive color method for detectingcracks in a metal body by selective corrosion of the surface portion within the cracks. Specifically, a color-forming aqueous acid indicating solution containing halide ions and a color-forming indicator is applied to the surface of the body to be tested. The solution is formulated so that it does not corrode to any significant extent the open surface portion of the body but is sufficiently reactive to corrode the surface portion within cracks resulting in the formation of metallic ions. The color-forming indicator is of the type which reacts with the thus formed metal ions to form a distinctive colored compound at the crack locations.

7 Claims, No Drawings This invention relates to a nondestructive method for detecting cracks in a metal body. In particular, the invention contemplates a process resulting in improved economy and versatility for inspection of metal bodies, whereby a clear, precise, reproducible record of the inspection results is rapidly obtained.

Metal fatigue causes 75 percent of all the serious engine failures in jet aircraft today. If adequate inspection techniques were available, these failures could be prevented. The fatigue process is a slow one; early in the process small microcracks are nucleated either at grain boundaries intersecting the surface, at hard precipitate particles at or near the surface, or other surface flaws. These microcracks grow at a very slow but accelerating rate leading to a large crack which will eventually cause catastrophic failure if it remains undetected. Since the fatigue crack grows slowly, there is usually ample opportunity to detect the crack during inspection periods before it grow to a critical size.

Current crack detection techniques rarely are able to detect cracks smaller than 3 mils in depth, and in most circumstances to mils is usually the lower limit of detectability. With the more traditional structural alloys and conservative design, a crack can grow to much larger than this detection threshold before it becomes critical in size. But the newer, high temperature alloys which have a much lower toughness and are designed for much higher stress levels, the critical crack size begins to approach the limit of detectability. Under these conditions detection techniques of the prior art are not adequate.

A typical detection process widely used in industry utilizes penetrants known in the art which include various liquids mixed with a dye in combination with a developer or blotting agent. The use of such a liquid penetrant generally comprises application of the penetrant to the surface of the metallic specimen followed by a soaking period of at least about 30 minutes to permit penetration into cracks, if any. The excess penetrant is then removed usually with a suitable solvent. A developer or blotting agent which contains a powder such as ground silica or tale, is then applied to the surface to provide a contrasting background to make the dye more visible and to blot the liquid dye out of the surface cracks by absorption. The pattern of blots shows the general location of cracks as well as other surface discontinuities.

These dye penetrant inspection procedures of the prior art have several drawbacks. They require a number of steps and are time consuming. The effectiveness of the liquid penetrant depends on its wetting properties. The penetrant must not only be able to flow into a crack but also be absorbed out of it by a developer, and therefore, microcracks almost closed at their mouths would not be detected in most instances. In addition, known liquid penetrants are limited regarding temperatures at which they may be used and begin losing their penetrating power at temperatures below 70 F. due to varying surface tension and flow characteristics. Also, the blotting action of the developer depends upon its absorption of the liquid dye penetrant. Since this absorbing action continues as long as liquid dye remains in contact with the developer, the pattern initially produced quickly becomes fuzzy and definition is lost in a matter of minutes. Moreover, the definition obtained in the dye pattern depends partly upon grain size of the developer, and since larger grain sizes produce poorer definition, developers of very uniform small grain size are required. In addition, the test specimens must be handled with care to prevent wiping off of the developer. Also, and most importantly, the mass of blots produced by conventional penetrants may be the result of normal voids or surface scratches, making the detection of structural microcracks difficult.

To avoid the use of developers, some prior art methods use a fluorescent penetrant. Typically, this comprises applying to the surface of the test body the luminescent penetrant, removing the penetrant remaining on the surface generally by solvent cleaning, and then inspecting the surface of the body in darkness under fluorescigneous radiation such as ultraviolet light or black light for the portion of the penetrant which had penetrated into the surface openings. The particular problem with this type of method is that many large castings and like bulky articles are not readily transported to locations which may be darkened as required for inspection. Such heavy bodies are also handled with considerable difficulty during the inspection procedure once the location is satisfactorily darkened. In addition, the sources of the required fluorescigenous radiations, almost exclusively high intensity mercury vapor arcs, are relatively expensive special equipment. Also, some The present invention overcomes the drawbacks of the prior art methods. It can be carried out in a single step in normal light in a matter of minutes, generally up to about fifteen minutes, and frequently, within shorter periods of time and does not use a developer or blotting agent. The present process is also particularly satisfactory for detecting cracks which are closed at their mouths. This is of particular importance since parts are inspected usually in an unstressed, cold condition which commonly leaves the cracks tightly closed.

Briefly stated, the process of the present invention comprises applying to the surface of a metal body to be tested a color-forming indicating solution which will form a distinctive colored compound at crack locations in the body. Specifically, the indicating solution is an aqueous acid solution containing halide ions and a color-forming indicator. The solution is formulated so as to be too weak to corrode the open surface portion of the body, but sufficiently strong to corrode the internal surface portion contained within the cracks resulting in the formation of metallic ions within the cracks. The color-forming indicator is of the type which reacts with the thus formed metal ions to produce a distinguishing colored compound.

The present process detects cracks in a metal body by selective corrosion of the surface portion within the cracks. By corrosion, it is meant herein the dissolution of the metal by its liquid environment. The bodies of metallic material which are useful in the present invention are those generally known in the art as the type that exhibit crevice corrosion. Representative of these materials are austenitic alloy steels such as stainless steels, iron-base super alloys and the nickel-chromiumiron alloys such as those sold under the trademark of lncoloy.

ln carrying out the process of the present invention, the surface of the metal body to be tested should be cleaned to reduce background indications so that a clearer and sharper indication of cracks can be produced. Conventional methods of cleaning metals can be used such as by wiping the surface with a suitable organic solvent to remove oils and grease or by mild grit blasting to remove adherent surface contaminants such 5 paint and scale.

The indicating solution is an electrolyte which is formulated so as not to corrode the open surface portion of the metal body, but to corrode the metal surfaces within cracks in the body. It is always an aqueous acid solution containing halide ions and a color-forming indicator. Its specific formulation is determinable empirically.

To provide the necessary acidity as well as the halide ions in forming the present indicating solution, a halogen acid is preferably used, i.e., hydrochloric, hydrobromic, hydroiodic or hydrofluoric acid. Hydrochloric acid produces the best results and is preferred. Upon application of the indicating solution to the surface to be tested, the halogen acid constituent acts to corrode the surface portion within cracks to cause the formation of metal ions. The resulting positively charged metal ions hydrolyze in the surrounding electrolyte in the crack, increasing its acidity and thereby increasing the corrosion rate within the crack. in addition, the positively charged metal ions formed within the cracks attract some of the halide ions of the solution which accelerates corrosion therein. The color-forming indicator reacts with the metal ions produced by corrosion in the crack to form a distinctive colored compound at the crack locations in a matter of minutes, generally up to about fifteen minutes and frequently within a shorter period of time. In contrast, acids such as sulfuric and nitric alone are not operable in the present process since they require an impracticably long period of time, usually within the range of about 20 hours, before sufficient corrosion of the metal within the crack occurs to produce a colored compound in accordance with the present invention.

The corrosive strength of the instant indicating solution is determined largely by the difference in the rate of corrosion between the open surface portion of the metallic material and the surface portion within a crack therein, i.e., it should be sufficiently strong to corrode the surface portion within a crack but not the open surface portion of the material to any significant extent. Since the acidity or the composition of the indicating solution is not significantly changed at the free surface portion, but the acidity of the solution does increase rapidly at the surface portion within a crack due to a number of factors such as, for example, a lack of oxygen within the crack, fairly weak acidic indicating solutions are satisfactory. The useful acidity and halide ion content of an indicating solution generally falls within a specific range for a particular metallic material so that the distinctive colored compound which indicates the crack locations is formed in a reasonable period of time, generally within about l minutes. The solutions of stronger acidity and/or higher halide ion content are preferably used where cracks of very small depth must be detected since these cracks corrode at a rate slower than deeper cracks. in addition, the stronger indicating solutions within a range are preferably used to locate cracks which are closed at their mouths. In this instance, the indicating solution enlarges the mouth of the crack by corrosion and flows in.

Specifically, for an iron base high temperature alloy such as A286(AlSl 660), an aqueous acid solution useful in forming the present color-forming indicating solution contains hydrochloric acid in an amount ranging from about 1 to about 20 percent by volume of the aqueous acid solution (i.e., about 0.1 to about 2 Normal) with the preferred hydrochloric acid concentration ranging from about 5 to about 15 percent by volume ofthe solution (i.e., about 0.5 to 1.5 Normal).

In one embodiment of the present invention, the a halide ion content of the indicating solution can be provided, or increased if desired, by the use of a soluble metal halide salt. This salt should be selected so that its metal ion does not form a colored compound with the color-forming indicator. Representative of such salts are alkali halide salts such as sodium chloride, potassium chloride and sodium iodide. Where a metal salt is used, the acidity of the indicating solution, if desired, may be provided by an acid other than a halogen acid. Representative of the acids useful with a metal halide salt are sulfuric acid and nitric acid.

The color-forming indicator is one which reacts with the metal ions formed by corrosion of the surface portion within the crack to produce a colored compound. For example, in the case of stainless steel, ferrous ions are formed by corrosion within the crack. One of the most satisfactory color-forming indicators for ferrous ions is potassium ferricyanide l- ,l-e(CN) which reacts with the ferrous ions to form a deep blue precipitate of ferrous ferricyanide known as Turnbulls blue. The reaction is as follows:

Additionally, ferric ion reacts with an ferrocyanide ions Representative color-forming indicators useful for specific metals are as follows:

azophenylarsonic acid Color-forming indicators for use with specific metal ions are well known in the literature. For example, an extensive list is disclosed in Feigl, F., Qualitative Analysis By Spot Tests, 3rd. ed., Elsevier Pub., NY. 1946. i

The amount of color-forming indicator is determinable empirically, and generally amounts ranging up to about four grams per cc. of the aqueous acid solution used in forming the indicating solution are satisfactory with amounts of about 0.5 gram to about 2 grams per 100 cc. of the aqueous acid solution being preferred.

The present process can be carried out in a number of ways. The color-forming indicating solution may simply be applied to the surface area of the metal body to be tested in a conventional manner such as from an eye dropper or other conventional means. After a short period of time, a distinctive colored compound will become visible at the crack locations. If desired, the part to'be tested may be immersed in the colorforming indicating solution, and after a short period of time, a colored compound will become visible at crack locations.

The colored compound which indicates crack locations is easily removed. it may be removed by simply wiping it off with a cloth or paper such as Kimwipe. Generally, it can be removed most readily by applying to the colored-compound carrying surface a complexing agent and then wiping off the complexcd compound. Typical complexing agents include ethylene diamine tetraacetic acid and Biguanide [H,NC( :Nl-l Nl-l.

A wetting agent can be included in the indicating solution to increase its penetrating power. Conventional wetting agents can be used and they may be anionic, cationic or nonionic. The wetting agent should not be decomposed or degraded by the indicating solution to any significant extent. The amount of wetting agent used is not critical and may vary within a wide range depending on the increased penetration produced by a specific amount. Generally, satisfactory results are obtained with amounts of wetting agent ranging from about 0.01 to about 2 percent by volume of. the indicating solution. Representative of the anionic wetting agents are sodium salts of organic sulfonates, especially alkylaryl sulfonates such as the sulfonates of dodecylbenzene, as for example, disodium-4- dodecylated oxydibenzenesulfonate. Other representative anionic surfactants include sodium alkyl-napththalenesulfonate, sodium N-methyl-N-oleyltaurate, sodium oleylisethionate and the sodium salt of sulfated nonyl phenoxypoly (ethyleneoxy) ethanol. Typical cationic surfactants include lauryltrimethylammonium chloride and octadecyltrimethylammonium chloride. Examples of nonionic surfac tants are polyethylene glycol lauryl ether and tris (polyoxyethylene) sorbitan monolaurate.

The present process is not destructive or harmful since the indicating solution usually causes a rounding and blunting of the crack at its base without deepening it. This has been illus trated by micrographs of transverse sections of cracks prior to application of the present process and after such application.

As disclosed in US. Pat. application Ser. No. 889,693 filed of even date herewith in the name of Michael F. Henry and assigned to the assignee hereof, and which be reference is made part of the disclosure of the present application, a particularly clear indication of cracks can be obtained by initially treating the surface of the metal to be tested to passivate the open surface portion. Specifically, roughness or normal voids in the surface frequently cause spotty background of varying color intensity in the test for cracks and makes detection of the cracks more difficult. To substantially reduce such a background, the surface to be tested is pre-treated preferably with the present color-forming aqueous acid indicating solution. Specifically, such pretreatment comprises applying a thin coating of the solution onto the surface, and after a specific short period of time, wiping off the coating. It is believed that the indicating solution passivates the background causing areas of the surface, i.e., forms a passive film thereon. Preferably, the coating is applied by saturating an absorbent material such as cotton with the indicating solution and rubbing it onto the surface. After a short period of time which can be determined empirically, and which generally ranges up to about -30 seconds, the coating is removed with a conventional material such as cloth or paper so as not to break to any significant extent the just-formed passive film. The solution coating should be wiped off before any significant amount of it enters a crack and thereby passivates the surface portion therein.

The invention is further illustrated by the following examples.

In the following examples, A286(AISI 660) an iron-base super alloy was used. This alloy is composed of Ni-26%, Crl5%, Mol.3%, Fe-balance, Ti2%, Al0.2% and B0.0l5%. This metallic material is a high temperature steel used in the aircraft industry as a disc alloy which is an application that undergoes considerable fatigue.

EXAMPLE 1 A bar of the A-286 alloy, 8 inches long, 0.5 inch wide and 0.125 inch thick was cut and a gage section was carefully sursolution was applied to a surface of the gage section of the barat room temperature from an eye dropper to cover the entire surface being tested. Within about 30 seconds a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surfaces tested.

EXAMPLE 2 The procedure used in this example was the same as that set forth in Example 1 except that another bar was used and the aqueous indicating solution was formed from 1 ml. of concentrated hydrochloric acid (38% conc.), 99 ml. of distilled water, 0.1 grams of potassium ferricyanide, and 0.1 ml. of a wetting agent sold under the trademark Aquet which is a nonionic alkyl aryl polyethylene glycol liquid. Within about l5-30 seconds, a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surface tested.

EXAMPLE 3 The procedure used in this example was the same as that set forth in Example 1 except that another bar was used and the aqueous indicating solution contained about 0.1 ml. of the wetting agent Aquet. Within about 30 seconds, a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. There was no visual evidence of any significant corrosion of the surface tested.

EXAMPLE 4 To determine if any chloride ions which might remain in the cracks of a treated metal specimen of the present invention had an effect on the life span of the specimen, eight bars of the same size as that set forth in Example 1 were cut and a gage section was surface ground in each in one direction. All the bars were fatigued by reverse bending to the same extent to cause microcracks therein in the direction normal to that of the grinding direction.

The aqueous indicating solution of Example 3 was then applied to a surface of the gage section of each bar at room temperature from an eye dropper to cover a substantial portion of the ground surface area. Within about 30 seconds, a deep blue precipitate occurred at the crack locations in each bar and was clearly visible in daylight. The blue precipitate was then removed from each bar by wiping it off with Kimwipe paper.

The eight bars were then placed in a clamped position under stresses in an atmosphere saturated with water vapor and maintained at a temperature of C. At the same time, for control purposes, an untreated bar of the same material size and gage section, was also fatigued in the same manner to the same extent, was also placed in the same atmosphere in the same clamped position.

After hours, all of the bars were-removed from the hot atmosphere and fatigued to failure by reverse bending. All of the bars failed at about the same number of bending cycles. This illustrates that the present detection process does not shorten the life of the metallic material.

EXAMPLE 5 The procedure used in this example was the same as that set forth in Example 1 except that another bar was used and the solution was formedfrom 20 ml. of concentrated hydrochloric acid (38% conc.), 80 ml. of distilled water, 2 grams of potassium ferricyanide and 0.1 ml. of the wetting agent Aquet. Within about 15-30 seconds, a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surface tested.

In copending US. Pat. application Ser. No. 889,699 entitled Anodic inhibitor-Color Method For Detecting Cracks ln Metal Bodies filed of even date herewith in the names of Louis F. Coffin, Jr. and Lyman A. Johnson and assigned to the assignee hereof there is disclosed a color method for detecting cracks in a metal body using a color-forming electrolyte which contains halide ions, 3 color-forming indicator and anodic inhibitor.

In copending U.S. Pat. application Ser. No. 889,694 entitled Pre-Passivation-Anodic Inhibitor-Color Method For Detecting Cracks In Metal Bodies filed of even date herewith in the name of Michael F. Henry and assigned to the assignee hereof there is disclosed a process combining pre-passivation with a color method for detecting cracks in metal bodies wherein a color-forming electrolyte containing halide ions, a color-forming indicator and an anodic inhibitor is used.

In copending US. Pat. application Ser. No. 889,695 entitled Method For Detecting Cracks in Metal Bodies filed of even date herewith in the names of Lyman A. Johnson and Michael F. Henry and assigned to the assignee hereof there is disclosed the selective corrosion of the surface portion within cracks to enlarge them sufficiently so that they can be detected by a conventional technique using a conventional liquid.

All of the above cited patent applications are, by reference, made part of the disclosure of the present application.

What is claimed is:

l. A color process for detecting cracks in a metal body wherein said body is of the type that undergoes crevice corrosion by selective corrosion of the surface portion within the cracks to produce a colored compound at said cracks which comprises applying to the surface of said metal body a colorforming aqueous acid indicating solution containing chloride ions and a color-forming indicator, said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to produce metal ions, and said color-forming indicator being reactive with the resulting metal ions to produce a colored compound, said process being carried out at room temperature within a period of minutes in the absence of an external potential.

2. A process according to claim 1 wherein said metal body is an austenitic steel.

3. A process according to claim 1 wherein said indicating solution is comprised of an aqueous solution of hydrochloric acid and a color-forming indicator.

4. A process according to claim 1 wherein the color-forming indicating solution contains a wetting agent.

5. A process for detecting cracks in an iron-base super alloy metal body by selective corrosion of the surface portion within the cracks to produce a blue precipitate at said cracks which comprises applying to the surface of said metal body a colorforming aqueous acid indicating solution comprised of an aqueous solution of hydrochloric acid and potassium ferricyanide, said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to produce ferrous ions and said potassium ferricyanide reacting with the resulting fer rous ions to produce a blue precipitate at said cracks, said process being carried out at room temperature within a period of 15 minutes in the absence of an external potential.

6. A process according to claim 5 wherein the solution is formed from about cc. of 5 to 10 percent by volume aqueous hydrochloric acid solution and about 1 to 2 grams of potassium ferricyanide.

7. A process according to claim 6 wherein the solution contains a wetting agent. 

2. A process according to claim 1 wherein said metal body is an austenitic steel.
 3. A process according to claim 1 wherein said indicating solution is comprised of an aqueous solution of hydrochloric acid and a color-forming indicator.
 4. A pRocess according to claim 1 wherein the color-forming indicating solution contains a wetting agent.
 5. A process for detecting cracks in an iron-base super alloy metal body by selective corrosion of the surface portion within the cracks to produce a blue precipitate at said cracks which comprises applying to the surface of said metal body a color-forming aqueous acid indicating solution comprised of an aqueous solution of hydrochloric acid and potassium ferricyanide, said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to produce ferrous ions and said potassium ferricyanide reacting with the resulting ferrous ions to produce a blue precipitate at said cracks, said process being carried out at room temperature within a period of 15 minutes in the absence of an external potential.
 6. A process according to claim 5 wherein the solution is formed from about 100 cc. of 5 to 10 percent by volume aqueous hydrochloric acid solution and about 1 to 2 grams of potassium ferricyanide.
 7. A process according to claim 6 wherein the solution contains a wetting agent. 